The average fuel economy of all cars sold in 2013 improved by 1 mile per gallon over 2012.

The estimated fuel economy listed on window stickers by the Environmental Protection Agency (EPA) was 24.8 mpg, according to researchers Michael Sivak and Brandon Schoettle of the University of Michigan’s Transportation Research Institute (UMTRI).

Sivak and Schoettle of UMTRI have been tracking the average sales-weighted fuel economy since 2007, when the average fuel economy of all cars sold was 20.8 mpg. Sales-weighted numbers are important to show what we’re buying, not just what automakers are making (we’re looking at you, California).

To clarify (or confound), there are two fuel economy numbers that are tracked and monitored by the EPA, the National Highway Traffic Safety Administration (NHTSA) and UMTRI: unadjusted sales averages and adjusted sales averages.

To put it as simply as possible, unadjusted fuel economy is based on the EPA’s lab testing cycle. Adjusted fuel economy is reduced by 20 percent to reflect real-world driving variables. Then there are credits for automakers reaching certain targets that influence the Corporate Average Fuel Economy (CAFE), but that’s straying really far from keeping it simple.

CAFE standards were instituted by the U.S. government in 1975 and finalized in 2012 to minimize emissions and maximize fuel efficiency. The CAFE target for average fuel economy over all light-duty vehicles is 54.5 mpg by 2025.

That commonly cited 54.5 mpg represents the unadjusted fuel economy; the unadjusted fuel economy in 2013 according to Sivak and Schoettle is 30 mpg. In 2012, it was 29.2 mpg.

The adjusted fuel economy of 24.8 is the common barometer because it’s the one we see on window stickers, but it should not be directly compared to the 54.5 mpg by 2025 target.

Though both measures have improved yearly, the unadjusted and adjusted mpg are slightly short of EPA targets. But targets are where the EPA wants us to be as a car-buying marketplace, not where they expect us to be. The target for 2013 was 31.1 mpg; the projected, or expected mpg, was 29.7 mpg.